This invention relates to back-up battery systems and, in particular, to back-up battery systems adapted for use in telephone plants or central offices.
In telephone plants, commercial AC power is used as a source of supply for rectifiers which convert the AC power to DC power for application to the switching equipment used in the plant. If there should occur an interruption or failure in the AC power, this will result in a corresponding failure in the converted DC power. Failure of the DC power, in turn, will result in failure of the switching equipment and an outage in telephone service.
In order to avoid such service outages, telephone plants usually employ back-up battery systems which are brought into service upon failure or interruption of the commercial AC power and/or converted DC power. Present day back-up battery systems typically comprise one or more rechargeable or secondary batteries which are permanently wired in parallel with the switching equipment or load. These batteries are designed to provide the necessary DC power to the load for a selected period of time within which the failure or interruption in primary power is expected to be restored. These batteries also serve as a low resistance path to ground during normal service, so as to prevent small load variations from affecting the supply of power.
Due to the characteristics of the switching equipment, the converted DC power supplied to the equipment must be maintained below a maximum voltage value and above a minimum voltage value. As a result, the voltage of the rechargeable secondary batteries in the back-up system must be maintained between these voltage values over the expected period of operation of the back-up system. Furthermore, it is also desired that the rechargeable batteries be floated at a voltage which is designed to maintain the float voltage of the individual cells of the battery at predetermined value considered necessary for extended battery life.
In a representative present day telephone plant, the switching equipment or load might draw a DC current of about 1150-1200 amps at a nominal or preselected DC negative load voltage of about -52 volts. The maximum and minimum negative voltage values sustainable by such a representative load, in turn, might be approximately -55 volts and -45 volts, respectively. For a load of this type, the standard back-up battery system might typically comprise four parallel rechargeable batteries each at a floated DC negative voltage of -52 volts and each capable of providing a current of 300 amps to the load. Each rechargeable battery, in turn, would then comprise 24 series connected standard battery cells in order to achieve a floated DC negative voltage per cell of approximately -2.17 volts, which is presently considered the most beneficial for extended battery life.
As is also known, such standard battery cells have a voltage characteristic which decreases sharply to an initial minimum value shortly after the cell is brought into operation. The voltage characteristic then increases to a plateau voltage and thereafter again decreases, this time gradually, over time.
Thus, in the above-mentioned representative back-up battery system the negative voltage of each cell of each battery string decreases to -1.93 volts within about 11/2 minutes of operation, then increases to a plateau voltage value of approximately -1.97 to -1.98 volts over the next 10-15 minutes of operation and thereafter decreases to -1.88 volts over the next 2 hours and 45 minutes. Accordingly, the 24 cells of secondary batteries, each start with an initial negative voltage of -52 volts, decrease in negative voltage to approximately -46 volts, then increase in negative voltage to approximately -48 volts and then again decrease in voltage to approximately -45 volts over a three hour period. The secondary batteries, therefore, stay within the required minimum and maximum load voltage requirements over a three hour back-up period, while combining to provide the current load requirement.
Back-up systems using the aforesaid standard cell 24 rechargeable batteries have worked successfully over many years. However, because of the aging of the switching equipment, there is a concern as to whether the standard system can continue to meet the minimum load voltage requirement which is believed to be increasing in value with time. Thus, in the representative system discussed above, instead of the switching equipment requiring a minimum negative voltage value of -45 volts, the aging of the equipment may increase the requirement to approximately -46 volts. Under these circumstances, the use of the aforesaid 24 cell back-up battery system may no longer accommodate the load requirements, due to the initial decrease or dip in voltage which, as above-indicated, decreases the negative battery voltage to close to -46 volts.
While simply increasing the voltage of each battery by one or two cells would appear to solve this problem, doing so also increases the float voltage requirements of the batteries, if the -2.17 volts per cell float standard is to be satisfied. Since the increased float voltage may exceed the maximum load voltage requirement, this simple modification is not a satisfactory solution to the problem.
It is, therefore, an object of the present invention to provide an apparatus and method for realizing an improved back-up battery system for the switching equipment in a telephone plant.
It is a further object of the present invention to provide a back-up battery system which is adapted to accommodate for the aging of the aforesaid switching equipment.